Besides nematic and cholesteric liquid crystals, optically active, tilted, smectic (ferroelectric) liquid crystals have also recently been used in commercial display devices.
Clark and Lagerwall have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1000 compared with conventional TN (xe2x80x9ctwisted nematicxe2x80x9d) cells (see, for example, EP-A 0 032 362). Owing to this and other favorable properties, for example the possibility of bistable switching and the fact that the contrast is virtually independent of the viewing angle, FLCs are basically highly suitable for areas of application such as computer displays.
For a more in-depth explanation of the technical requirements of FLCs, see European Patent Application 97118671.3 and DE-A 197 48 432.
Benzothiazole derivatives are disclosed in JP-A 09059266 as components of liquid-crystalline mixtures.
Benzothiazoles having one or more F substituents have been described as constituents of complex structures having a pharmacological action, for example in
EP-A-0 714 893,
EP-A-0 795 550, and
EP-A-0 733 365.
These publications do not reveal any suitability of mono- or difluorinated benzothiazoles as components of liquid-crystalline, in particular smectic mixtures and a part of these benzothiazoles.
Since the development, in particular of ferroelectric liquid-crystal mixtures, can in no way be regarded as complete, the manufacturers of displays are interested in a very wide variety of components for mixtures, partly because only the interaction of the liquid-crystalline mixtures with the individual components of the display device or of the cells (for example the alignment layer) allow conclusions to be drawn on the quality of the liquid crystalline mixtures too.
It has now been found that fluorinated benzothiazoles of the formula (I), even when admixed in small amounts, have a favorable effect on the properties of liquid-crystal mixtures, in particular chiral smectic mixtures, for example regarding the dielectric anisotropy and/or the melting point, but also regarding the switching behavior.
The invention relates to the use of compounds of the formula (I) below in liquid-crystal mixtures, preferably smectic and nematic liquid-crystal mixtures, particularly preferably ferroelectric liquid-crystal mixtures. Particular preference is given to use in ferroelectric liquid-crystal mixtures operated in inverse mode or in displays having active matrix elements; very particular preference is given to use in ferroelectric liquid-crystal mixtures operated in inverse mode.
The invention furthermore relates to liquid-crystal mixtures, preferably smectic and nematic liquid-crystal mixtures, particularly preferably ferroelectric liquid-crystal mixtures, which comprise one or more compounds of the formula (I) below 
where the symbols and indices have the following meanings:
X1 and X2, independently of one another, are xe2x80x94CHxe2x80x94, xe2x80x94CFxe2x80x94, the xe2x80x94(R1)xe2x80x94Cxe2x80x94 group or the xe2x80x94(R1xe2x80x94A1xe2x80x94M1)Cxe2x80x94 group
Y1 and Y2, independently of one another, are hydrogen or fluorine, but are not simultaneously fluorine
with the provisos that:
x1) X2 is the xe2x80x94(R1)Cxe2x80x94 group or the xe2x80x94(R1xe2x80x94A1xe2x80x94M1)Cxe2x80x94 group, and X1 is xe2x80x94CFxe2x80x94, Y1 is F and Y2 is H, or
x2) X1 is the xe2x80x94(R1)Cxe2x80x94 group or the xe2x80x94(R1xe2x80x94A1xe2x80x94M1)Cxe2x80x94 group, and X2 is xe2x80x94CFxe2x80x94, Y2 is F and Y1 is H, or
x3) X2 is the xe2x80x94(R1)Cxe2x80x94 group or the xe2x80x94(R1xe2x80x94A1xe2x80x94M1)Cxe2x80x94 group, and X1 is xe2x80x94CHxe2x80x94, Y1 is H and Y2 is F, or
x4) X1 is the xe2x80x94(R1)Cxe2x80x94 group or the xe2x80x94(R1xe2x80x94A1xe2x80x94M1)Cxe2x80x94 group, and X2 is xe2x80x94CHxe2x80x94, Y2 is H and Y1 is F,
x5) X2 is the xe2x80x94(R1)Cxe2x80x94 group or the xe2x80x94(R1xe2x80x94A1xe2x80x94M1)Cxe2x80x94 group, and X1 is xe2x80x94CHxe2x80x94, Y1 is F and Y2 is H
R1 and R2, independently of one another, are a straight-chain or branched alkyl radical (with or without asymmetrical carbon atoms) having 1 to 20 carbon atoms, where
a) one or two non-terminal xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94C(xe2x95x90O)xe2x80x94 and/or xe2x80x94Si(CH3)2xe2x80x94, with the proviso that two adjacent xe2x80x94CH2xe2x80x94 groups cannot be replaced by heteroatoms, and/or
b) one or more xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94 and/or xe2x80x94Cxe2x89xa1Cxe2x80x94, and/or
c) one xe2x80x94CH2xe2x80x94 group may be replaced by cyclopropane-1,2-diyl, cyclobutane-1,3-diyl, cyclopentane-1,3-diyl, bicyclo-[1.1.1]pentane-1,3-diyl or cyclohexane-1,4-diyl, and/or
d) one or more H atoms may be replaced by F, and/or
e) in the case of a branched alkyl radical containing asymmetrical carbon atoms, the asymmetrical carbon atoms either have xe2x80x94CH3, xe2x80x94OCH3, xe2x80x94CF3, F, CN and/or Cl as substituents, or are incorporated into a 3xe2x80x94 to 7-membered ring, in which, in addition, one or two non-adjacent xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94Oxe2x80x94 and one xe2x80x94CH2xe2x80x94 group may be replaced by xe2x80x94OC(xe2x95x90O)xe2x80x94
A1 and A2, independently of one another, are phenylene-1,4-diyl, unsubstituted, monosubstituted or disubstituted by CN or F, phenylene-1,3-diyl, unsubstituted, monosubstituted or disubstituted by CN or F, cyclohexane-1,4-diyl, in which one or two H atoms may be replaced by CN and/or CH3 and/or F, 1-cyclohexene-1,4-diyl, in which one H atom may be replaced by F, 1-alkyl-1-silacyclohexane-1,4-diyl, pyridine-2,5-diyl, unsubstituted or monosubstituted by F, pyrimidine-2,5-diyl, unsubstituted or monosubstituted by F, 1,1xe2x80x2-biphenyl-4,4xe2x80x2-diyl, unsubstituted, monosubstituted or disubstituted by F, or 1,1xe2x80x2-phenylcyclohexyl-4,4xe2x80x2-diyl, in which the phenyl moiety is unsubstituted, monosubstituted or disubstituted by F
M1 is undirected and is xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94OC(xe2x95x90O)Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94(CH2)nxe2x80x94, xe2x80x94OC(xe2x95x90O)CH2CH2xe2x80x94, xe2x80x94OCH2CH2CH2xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond
n is from 1 to 6.
Undirected is intended to mean that, in the case of asymmetrical groups, mirror and mirror image of the group can be employed.
According to one embodiment, the proviso x5) does not apply.
The symbols and indices in the formula (I) preferably have the following meanings:
R1 and R2, independently of one another, are preferably a straight-chain or branched alkyl radical (with or without asymmetrical carbon atoms) having 2 to 18 carbon atoms, where
a) one xe2x80x94CH2xe2x80x94 group may be replaced by xe2x80x94Oxe2x80x94, and/or
b) one or more H atoms may be replaced by F.
R1 and R2, independently of one another, are particularly preferably a straight-chain or branched alkyl radical (with or without asymmetrical carbon atoms) having 3 to 16 carbon atoms, where
c) one xe2x80x94CH2xe2x80x94 group may be replaced by xe2x80x94Oxe2x80x94.
A1 and A2, independently of one another, are preferably phenylene-1,4-diyl, phenylene-1,3-diyl, 1,1xe2x80x2-biphenyl-4,4xe2x80x2-diyl or 1,1xe2x80x2-phenylcyclohexyl-4,4xe2x80x2-diyl.
A1 and A2, independently of one another, are particularly preferably phenylene-1,4-diyl.
n may have the value 2, 4, 5 or 6.
The following compounds of the formulae (I-1) to (I-5) are particularly preferably employed: 
where R1 and R2 have the above-mentioned meanings and preferences.
According to one embodiment of the invention, the formula (I-5) is not one of the preferred compounds.
The invention also relates to the fluorinated benzothiazoles of the formula (1) with the exception of the proviso x4) or the formula (I-4).
The compounds according to the invention and employed according to the invention are prepared by methods known per se from the literature, as described in standard works on organic synthesis, for example Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart.
However, it may prove necessary to vary or modify the literature methods for the requirements of mesogenic units, since, for example, functional derivatives having long ( greater than C6) alkyl chains frequently have lower reactivity than, for example, the methyl and ethyl analogs.
Particular reference is made in this connection to the following references, which describe in general terms the synthesis of benzothiazole derivatives, but which do not reveal to the person skilled in the art the fluorinated benzothiazoles according to the invention and their suitability as components of liquid-crystalline mixtures:
Reference 1: Mitsumoto et al., Tokyo Kogyo Koto Senmon Gakko Kenkyu Hokokushu (1993), 25, 73-5 [CAN 121:57382].
Reference 2: Chen et al., Phosphorus, Sulfur Silicon Relat. Elem. (1992), 68(1-4), 205-10 [CAN 117:69775].
As far as the linking of functional derivatives of the fluorinated benzothiazoles with other liquid-crystal-specific units is concerned, express reference is made to DE-A 197 48 432, which gives a list of methods customary to the person skilled in the art.
The liquid-crystal mixtures according to the invention generally comprise from 2 to 35 components, preferably from 2 to 25 components, particularly preferably from 2 to 20 components.
They generally comprise from 0.01 to 80% by weight, preferably from 0.1 to 60% by weight, particularly preferably from 0.1 to 30% by weight, of one or more, preferably from 1 to 10, particularly preferably from 1 to 5, very particularly preferably from 1 to 3, compounds of the formula (I).
Further components of liquid-crystal mixtures which comprise compounds of the formula (I) are preferably selected from known compounds having smectic and/or nematic and/or cholesteric phases. Further mixture components which are suitable in this context are listed, in particular, in international patent application PCT/EP 96/03154 and DE-A 197 48 432, which are expressly incorporated herein by way of reference.
The mixtures according to the invention can in turn be used in electro-optical or fully optical elements, for example display elements, switching elements, light modulators, elements for image processing and/or signal processing, or generally in the area of nonlinear optics.
The invention furthermore relates to a switching or display device, preferably a device utilizing the smectic phases in the region of the working temperature. Particular preference is given to ferroelectric switching and/or display devices operated in normal or inverse (xcfx84Vmin) mode (see, for example, J. C. Jones, M. J. Towler, J. R. Hughes, Displays 1993, 14, No. 2, 86-93; M. Koden, Ferroelectrics 1996, 179, 121-129).
Particular preference is likewise given to ferroelectric switching and/or display devices containing active matrix elements (see, for example, DE-A 198 22 830).
The present application cites various documents, for example in order to illustrate the technical background to the invention. All these documents are expressly incorporated herein by way of reference.
The invention is explained in greater detail by the examples below without this being intended to represent a restriction thereto.